Electric machines such as motors and generators, are generally used because they are extremely rugged, reliable, easy to control, and in particular have a high torque capacity and high power density ratings. Switched reluctance machines operate on the principle that current traveling in stationary coils or windings of a stator produces a rotating magnetic field which in turn interacts with a rotor occupying the space where the rotating magnetic field exists. The magnetic teeth of the rotor react with the rotating magnetic field to produce a rotational force.
Heretofore, it was believed that there was a fundamental limit to torque density in such machines. Although flux density is limited by material considerations, while current density is limited by (1) heating, (2) machine reactance, and (3) the fact that too much current density produces localized magnetic saturation, the present invention optimizes the configuration of the rotor and stator elements so that the machine output can be increased without substantially increasing the volume of the machine. Conventional belief in the design of electric machines is that power density is limited and the only way to increase power output is to increase the volume of the machine.
Switched reluctance motors operate on the principle of unipolar current, i.e., the current flows only in one direction in the windings regardless of whether positive or negative torque is required. This principle requires only one switch to be in series with each winding in each stator element. The turning on or off of this switch regulates the flow of current in the winding. It should be noted that in the motor literature an individual winding of a stator element is sometimes generally referred to as a “phase”. In the context of a unipolar current electrical machine the term “phase” is somewhat analogous to a phase of a multiphase alternating current motor.
The primary object of this invention is to provide a switched reluctance electrical machine having a high torque capacity for a given machine volume. A second object of this invention is to provide such a switched reluctance electrical machine in which the commonly accepted limit to torque density in electrical machines is overcome by utilizing the same magnetic flux among one or more parallel air gaps. A third object of this invention is to provide a switched reluctance electrical machine in which the magnetic flux is passed through multiple air gaps, interacting with a rotor element at each air gap, thereby increasing the torque density for a give volume of the machine. A fourth object of this invention to provide such an electrical machine in which force density is increased substantially by the number of air gaps present in the machine but the overall machine transverse dimension is increased only by a smaller factor because the magnetic return path remains nearly constant. A fifth object of this invention is to provide a fault tolerant switched reluctance electrical machine which can continue to operate even when one or more winding faults have been detected. A sixth object of this invention is to provide a method of manufacture for such a switched reluctance electrical machine.